1. Field of the Invention
This invention relates to electrophotographic image forming apparatus, and more particularly to cleaning devices for removing residual toner and debris from a charge retentive surface of an image forming apparatus.
2. Description of Related Art
In electrophotographic applications such as xerography, a charge retentive surface of a photoreceptor is electrostatically charged, and exposed to a light pattern of an original image to be reproduced, to selectively discharge the photoreceptive surface in accordance therewith. The resulting pattern of charged and discharged areas on that surface form an electrostatic charge pattern (an electrostatic latent image) conforming to the original image. The latent image is developed by contacting it with a finely divided electrostatically attractable powder referred to as toner. Toner is held on the image areas by the electrostatic charge on the surface. Thus, a toner image is produced in conformity with a light image of the original beam reproduced. The toner image may then be transferred to a substrate (e.g., paper), and the image affixed thereto to form a permanent record of the image to be reproduced. The process is well know, and is useful for light lens copying from an original, and printing applications from electronically generated or stored originals, where a charged surface may be discharged in a variety of ways. Ion projection devices where a charge is imagewise deposited on a charge retentive substrate operate similarly.
Multi-color electrophotographic printing is substantially identical to the foregoing process of black and white printing. However, rather than forming a single latent image on the photoreceptor, successive latent images corresponding to different colors are recorded thereon. Each single color electrostatic latent image is developed with toner of a color complementary thereto. This process is repeated in a plurality of cycles for differently colored images and their respective complementarily colored toner. Each single color toner image is transferred to the copy sheet in superimposed registration with the prior toner image. This creates a multi-layered toner image on the copy sheet. Thereafter, the multi-layered toner image is permanently affixed to the copy sheet as described above to create a color copy. The developer material (toner) may be a liquid material or a powder material.
Although, a preponderance of the toner forming the image is transferred to the paper during transfer, some toner invariably remains on the charge retentive surface of the photoreceptor, it being held thereto by relatively high electrostatic and/or mechanical forces. Additionally, paper fibers, toner additives, Kaolins and other debris have a tendency to be attracted to the charge retentive surface. It is essential for optimum imaging that the toner and debris remaining on the surface be cleaned thoroughly therefrom.
Blade cleaning is a highly desirable method for removal of residual toner and debris (hereinafter, collectively referred to as "toner") from a photoreceptor. In a typical application, a relatively thin elastomeric blade member is provided and supported adjacent to and transversely across the photoreceptor surface with a blade edge chiseling or wiping toner from the surface. Subsequent to release of toner from the surface, the released toner accumulating adjacent to the blade is transported away from the blade area by a toner transport arrangement, or by gravity. Unfortunately, blade cleaning suffers from certain deficiencies, primarily resulting from the frictional sealing contact which must be maintained between the blade and the photoreceptor surface. One common problem is the build up of material on the photoreceptor referred to as "comets". These comet defects are formed from high friction between the cleaning blade and the photoreceptor resulting in small particles becoming permanently attached with high adhesion forces to the photoreceptor. Frequently, toner additives which are not easily removed from the photoreceptor by these cleaning blades are melted by these high frictional forces, and permanently bonded to the photoreceptor. Additional particles continue to accumulate behind the initial "comet heads" and can form a 1-5 millimeter long comet tail attached to the photoreceptor. These comets can cause copy quality defects in the form of spots on the copy sheet in background areas.
FIG. 1 illustrates the manner in which comets are formed on the charge retentive surface of a photoreceptor 20. Photoreceptor 20 moves in the direction indicated by arrow 22. Toner particles 90 remaining on photoreceptor 20 after transfer of the toner image from the photoreceptor to a substrate (paper) are removed from the photoreceptor by a primary cleaning device such as, for example, a cleaning blade 110. Cleaning blade 110 is arranged at a low angle to the photoreceptor 20. Most of the toner particles accumulate upstream of blade 110 in the area denoted by reference numeral 93. This accumulated toner is then transported away by a toner transport arrangement or gravity. However, as illustrated in FIG. 1 the tip of blade 110 can become bent due to the movement of photoreceptor 20, and the high friction forces generated between blade 110 and photoreceptor 20. At this time, some toner particles 91 can become located between the bent portion of blade 110 and the photoreceptor 20, where they are pressed into the photoreceptor with a high force. This causes these toner particles to melt and become permanently attached to the photoreceptor. Additional toner particles build up in front of these bonded toner particles with subsequent photoreceptor rotation and are also pressed into the photoreceptor 20 with a high force, causing the "comet tails" 92 to grow.
Current technology for controlling comets requires the addition of specific additives to the dry ink material that can reduce cometing in specific machine applications. However, additives which work in one type of machine are not necessarily effective in eliminating comets when used with other machines.
Accordingly, a need exists for a photoreceptor cleaning device which prevents comets from forming on the photoreceptor. Preferably, this cleaning device should prevent high friction forces from being generated between a primary cleaning member and the photoreceptor to prevent toner particles from being pressed with high forces against the photoreceptor.
A number of cleaning apparatus for photoreceptors which employ the combination of a brush and a cleaning blade are known.
U.S. Pat. No. 4,989,047 to Jugle et al discloses a photoreceptor cleaning apparatus for the reduction of agglomeration-caused spotting. A thin scraper member arranged at a low angle to the photoreceptor is provided as a secondary cleaning device to a rotating negatively biased fiber brush which contacts the surface of the photoreceptor upstream of the blade to remove most of the adhering toner particles. The rotating brush removes the preponderance of toner from the photoreceptor, and the blade removes any toner agglomerates formed on the photoreceptor by the agglomeration of toner, and toner and debris.
U.S. Pat. No. 4,364,660 to Oda discloses a photoreceptor cleaning system having a cleaning blade which removes toner from a photoreceptor. A fur brush located upstream of the cleaning blade acts as a toner recovery mechanism to recover toner removed from the photoreceptor by the cleaning blade. The brush is made from synthetic resin filaments having a diameter of 0.1 mm. The brush rotates in a direction opposite from the photoreceptor to direct toner toward the blade.
U.S. Pat. No. 4,451,139 to Yanagawa et al discloses a cleaning apparatus for a photoreceptor which includes an elastic polyurethane cleaning blade located downstream of a rotating fur brush with respect to the rotation direction of the photoreceptor.
U.S. Pat. No. 3,918,808 to Narita discloses a photoreceptor developing and cleaning station wherein a cleaning blade is placed in a developing station which uses a magnetic brush to apply toner to a photoreceptor. Two complete revolutions of a photoreceptor are required to perform a single copying operation. During a first revolution, the blade is retracted. After transfer of a toner image from the photoreceptor to a copy sheet, the blade is contacted with the photoreceptor to remove residual toner from the photoreceptor.
U.S. Pat. No. 3,947,108 to Thettu et al discloses a photoreceptor cleaning system wherein a blade acts as a primary cleaning member. A brush located downstream of the blade removes a residual film from the photoreceptor not removed by the blade. The brush is abrasive and made from cotton or plastic fibers.
U.S. Pat. No. 4,875,081 to Goffe et al discloses a blade member for cleaning a photoreceptor wherein an A.C. voltage is applied to the cleaning blade. Use of the A.C. voltage eliminates the need to bias the blade against the photoreceptor with a high frictional force and thus, eliminates impaction of toner on the photoreceptor surface.
U.S. Pat. No, 4,835,807 ;to Swift discloses a cleaning brush for an electrostatographic reproducing apparatus which has electroconductive fibers of nylon filamentary polymer substrate having finely divided electrically conductive particles of carbon black suffused therein.
None of these patents disclose the present invention because they do not provide a rotating brush that abrades a photoreceptor upstream of a primary cleaning device for cleaning the photoreceptor.